For me the need to forgive is usually but not always associated with a resentment.
Say a certain person did me wrong. Whether it was real or simply perceived to have happened makes no difference because perception is reality. I get a resentment toward that person. My experience in AA is that the only way I can get better and get rid of the resentment is to forgive them and let it go. It does not matter the situation or the severity of the consequences thier action caused in my life. If I want to get better I have to forgive and let it go.
There are exceptions. Say I discover my young son lying to me. Do I resent him? Of course not, but I am disappointed in his behavior. To me there is a world of difference between disappointment and resentment. The only way to improve this situation is to have a loving talk with him explaining my disappointment and then forgive him. Forgiving in this regard also means to let it go. I don't bring it up in the future and say remember when you lied. If I do this then I have not really forgiven nor have I really let it go.
Anger is an acid that can do more harm to the vessel in which it is stored than to anything on which it is poured.